In the field of papermaking, various methods of improving the wet strength of paper are known by retaining wet strength agents to the cellulosic fibres in the pulp suspension while forming the paper. The wet strength of a paper relates to its ability to maintain physical integrity and to resist tearing, bursting, and shredding under use, especially under wet conditions. A further important property of wet strengthened paper is the softness, especially for tissue paper or the like. The softness can be described as the tactile sensation perceived when holding or rubbing a paper across the skin.
WO01/21890 discloses a method of modifying cellulose fibres to provide high wet strength to a paper. This method, however, involves adding electrolyte to the pulp suspension and treating it at a temperature of at least 100° C. which restricts flexibility and the use of this process.
The present invention intends to provide an energy-efficient and simple method for producing paper with increased wet strength and softness as well as other advantageous properties imparted by the modification of the fibres. It is a further object of the present invention to provide a method which can be used with conventional existing equipment and machinery.